It is a type of glass known as soffiati. Soffiati glass is impossibly thin. It was most popular in the 1920-30s in Venice, though it is still made. Most soffiati I have seen is unmarked -- even the Venini soffiati, since much was made before Venini started stamping their glass. Companies known for fine soffiati like this include Venini, Cappellin, and Zecchin-Martinuzzi (different Zecchin), though there were others. A good place to start is with a flashlight and some good magnifiers to see if you can find any marks at all. Sometimes a surprise "Made in Italy" may hop out.